Multiple-input multiple-output (MIMO) technology is a technology for enhancing transmission and reception efficiency of data using multiple transmission antennas and multiple reception antennas instead of using one transmission antenna and one reception antenna. When a single antenna is used, the receiving side receives data through a single antenna path, but if multiple antennas are used, the receiving end receives data through various paths. Hence, the transmission rate of data and the amount of data transmitted may be enhanced, and coverage may be increased.
In a multiple antenna wireless communication system, a rank indicator (RI), a precoding matrix index (PMI), channel quality information (CQI), etc. are defined as information, which is fed back to the receiving end (e.g., a terminal) and the transmitting end (e.g., a base station). Such feedback information may be called channel state information (CSI). The PMI and RI, which is preferred by the terminal, may be fed back to the base station. Here, if used in a base station in a given channel state, the PMI and RI, which is preferred by the terminal, corresponds to the downlink transmission rank value and precoder information which may have the highest transmission rate. Further, the CQI is calculated on the basis of the RMI/RI reported by the terminal, and corresponds to the modulation and coding scheme (MCS) level applied to downlink transmission. Here, in the case in which the precoder information and rank value according to the PMI/RI reported by the terminal is used, the CQI may indicate an MCS level which provides an allowable packet error rate.
Channel measurement should be performed for the report of the CSI, and channel measurement may be performed using a reference signal (RS). In a conventional multi-antenna wireless communication system, channel measurement may be performed using a reference signal (e.g., a common RS (CRS)) transmitted for each subframe.